1. Field of the Invention
The present invention relates to an air compressor for creating compressed air for driving a pneumatic tool such as a nailing machine.
2. Description of the Related Art
Generally, an air compressor for driving a pneumatic tool is designed to convert rotary motion of a motor into reciprocatory motion of a piston inside a cylinder via a crankshaft such that air sucked in from the suction valve of the cylinder is compressed by the reciprocatory motion of the piston. The air compressed within the cylinder is discharged into an air tank through a pipe from an exhaust valve and stored in the tank. A pneumatic tool such as a nailing machine operates by making use of the compressed air stored in the tank.
Such a conventional air compressor may be, in a rare case, of the stationary type having a large-sized air tank having a large capacity of creating compressed air. However, generally, air compressors are transported into building sites and run. Therefore, many of air compressors are portable types having relatively small-sized air tanks. Hence, there is a demand for an air compressor which has an air tank delivering a small amount of compressed air, i.e., relatively small capacity of producing compressed air, and which is minimal in size and has excellent portability.
Furthermore, an air compressor has a function of stopping the motor when the pressure inside the tank reaches a certain value that is an upper limit for safety reasons. Also, when the pressure inside the tank has decreased below a certain value that is a lower limit because of use of a pneumatic tool, the compressor restarts the motor. This function is accomplished by detecting the pressure inside the air tank by a pressure sensor and controllably turns on and off the power supply of the motor in response to the signal from the sensor.
FIG. 5 shows the pressure inside a tank for storing compressed air during operation of the prior art air compressor. In the graph of FIG. 5, the vertical axis indicates the pressure P (MPa) inside the tank, while the horizontal axis indicates the time T (min). Poff indicates a pressure at which the motor is stopped. Pon indicates a pressure at which the motor is restarted. PL indicates a work limit pressure at which the pneumatic tool such as a nailing machine is made inoperative due to a decrease of the pressure inside the tank.
In this prior art technique, the motor restart pressure Pon is so set that a certain extent of difference is produced between this pressure Pon and the motor stop pressure Poff. For example, this value is set such that Pon<(0.9×Poff), for the following reason. The motor comes to a stop at the point of the motor stop pressure Poff. Then, the pressure inside the tank mildly drops because of decrease of the temperature inside the tank and air leakage. Therefore, where the difference between Poff and Pon is small, the motor repeatedly and frequently starts and stops alternately even if no pneumatic tool is used. This state of oscillation should be prevented.
In FIG. 5, if the motor is started under the condition where no compressed air is consumed and the pressure inside the tank is zero, the pressure inside the tank rises. When the motor stop pressure Poff is reached at point a, the motor comes to a stop. If a pneumatic tool consuming only a small amount of compressed air is used continuously immediately after the stop of the motor, the pressure inside the tank drops relatively mildly and reaches the motor restart pressure Pon at point b, at which time the motor is restarted. The pressure inside the tank again increases and reaches the motor stop pressure Poff at point c, at which time the motor comes to a stop. Immediately after stop of the motor at the point c, if a pneumatic tool consuming a large amount of compressed air is used continuously, the pressure inside the tank drops rapidly and reaches Pon at point d. At this point, the motor is restarted. In the case of a small-sized portable compressor, however, the amount of created compressed air cannot catch up with the amount of consumption and so the pressure inside the tank keeps dropping. At last, the pressure reaches the work limit pressure PL at point e, where the work can no longer be continued. In this case, the work is interrupted, and the next work must be performed after waiting for the pressure inside the tank to rise.
In the prior art, a technique consisting of setting the motor stop pressure Poff to a higher value to increase the amount of usable compressed air has been adopted to solve the above-described problem.
For example, in an air compressor having a tank capacity of 10 liters, if the work limit pressure PL is 2.0 MPa, the amount of usable air (converted into atmospheric pressure) varies (or, increases) from 100 liters to 150 liters when the motor stop pressure Poff is increased from 3.0 MPa to 3.5 MPa. That is, the amount of usable air can be increased by 50% by increasing the motor stop pressure Poff from 3.0 MPa to 3.5 MPa.